In the lactating mammary gland, iodide is concentrated up to 36-fold in milk compared with the plasma iodide concentration, due to stimulation of the sodium/iodide symporter (NIS). Some hormone-dependent breast cancers also concentrate iodide, up to 7-fold, but iodide uptake in breast cancer must be enhanced to make radioiodine treatment possible, analagous to the TSH stimulation required for uptake of radioiodine into thyroid cancer. All-trans retinoic acid (RA) treatment stimulates iodide uptake, MS mRNA, and MS protein in MCF-7 cells, an estrogen receptor (ER) positive human breast cancer cell line, in a time and dose dependent fashion. No RA-induction of iodide uptake is seen in an ER-negative breast cancer cell line, MDA-MB 231, or a normal breast-derived cell line, MCF-12A. An in vitro clonogenic assay demonstrated selective toxicity of radioiodine following RA stimulation of MCF-7 cells. MCF-12A cells have abundant MS protein, but no functional iodide uptake. MS regulation differs significantly in the breast compared to the thyroid, and differs in normal breast and breast cancer cell lines. We propose to study the regulation of iodide transport in breast cancer cell lines compared to normal breast and thyroid cell lines, with the goals of optimzing iodide uptake and selectively targeting breast cancer cells. Specific aims include: 1. Determine the mechanism of RA-mediated transcriptional regulation of the NIS gene in breast-derived cells utilizing selective retinoid agonists and cell lines with a range of endogenous RAR and RXR expression. 2. To determine the characteristics of NIS protein expression, subcellular localization, and kinetics that are associated with maximal function of NIS in breast-derived cell lines and those features that distinguish functional iodide uptake among cell lines that express NIS protein. 3. Utilize an in vitro model to optimize RA-stimulated radiation-mediated cell killing in breast cancer cells. 4. Develop in vivo models to determine the efficacy and specificity of RA-stimulated iodide uptake into breast cancer and determine the influence of enhancers and radiation sensitizers. RA raioiodide uptake may be useful for diagnosis and treatment of some differentiated breast cancer.